Bushing Tolerances 5

[Aconnell]

Hello, I am an junior engineer at a smaller repair and overhaul company and I am writing a process specification for the installation of shrink fit and press fit bushings in mettalic structure. I am having trouble determining what interferences should be used. I was wondering if anyone knows of any industry specifications (possibly AMS or MIL Spec) that I could reference.

I would greatly appreciate any feed back.

Alex C

 

[ewh]

Try the Machinery's Handbook. It has listings for various fit designations.

 

[HSThompson]

I have an excellent bushing installation spec, drawings, and process description that I have used for years. I've found that when used, freeze fit bushings can be installed with light tapping and perfect fit every time.

I tried machinerys' handbook a few times and found the tolerances too wide. Sometimes the bushings fall in and don't hold and somtimes you beat on them so hard you damage the bushing or even worse, damage the hole making the situation that much more compicated.

 

[Aconnell]

Thank you everyone for your input, it is very much appreciated.

Thanks again.

 

[edbgtr]

Alex
Take a look at the bushing and other products on offer from FTI at

http://www.fatiguetech.com

for their mandrel installed bushes, and other products, it may change your life.
PS: I have no interest in this company other than what I have seen and heard.
Good luck,
Ed

 

[Aconnell]

HSThompson I would really appreciate it if you wouldn't mind sending me that installation spec, but I'm not sure how to contact you outside of this site.

 

[HSThompson]

Sorry it's taken so long for me to respon. We gotta take some vacation sometime.

Anyway, I'm not sure either, I recieved an e-mail from this site warning about not giving out e-mail addresses.

Go figure? I can paste the text here, but without the graphics, it's pretty much useless. I don't see what's wrong with posting your address, home or work, and I could snail-mail you a copy on disc?

Or we could meet outside this site and post e-mails there? like some yahoo stock chat room?

or we could post the e-mail address broken up in separate posts here so the spammer software wouldn't pick it up?

I don't know, these guys need to come-up with a way to transfer files and graphics back and forth allot easier.

 

[CoryPad]

HSThompson,

The reason you were warned about sharing your email address is because there are unscrupulous people on the Internet that will scan this site looking for addresses to send spam. If you are willing to take the risk, then share your email address. Another reason to dissuade people from email is that if you share information in this thread, everyone benefits. One way to minimize your risk is to use a text-based email description like name at yahoo dot com, to avoid auto recognition systems. Lastly, your comments regarding file transfers and graphics should be shared with the site management using the Contact Us link in the upper right corner of the page.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Aconnell]

Hello HSThompson, my email is Alex(dot)Connell(at)l-3com(dot)com. Hopefully the site administration does not have a problem with me posting my email in this format. Thanks again for your help.

Alex

 

[Sparweb]

CoryPad,

Sharing data by e-mail is well within the spirit of the forum, in my opinion. Anyone following the thread who is sufficiently interested can ask for the document.

I have a hotmail account set up specifically for the purposes of handling these kinds of messages. It's free and handy to have. Even so, I'm still not getting much spam, so the plan is working better than I expected. I also have a personal e-mail account that I (almost) never give out.

HST, do you mind also sending me a copy? sparwebAThotmailDOTcom



Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[Lindner]

The SRM or CMM should give the interference you need, it normaly varies between .001 to .0015.

 

[737eng]

For BAC aircraft the SRM chap 51-20-10 provides guidelines for freeze plug installations and I believe the tolerance is 1.0035-1.0040 times the hole diameter.

 

[chuck1]

Two excellent sources for bushing tolerances/fits are ANSI B4.1, American National Standard for Preferred Limits and Fits for Cylindrical Parts, and, if you are working with British aircraft, BS 1916 (inch) or BS 4500 (metric).

 

[edmeister]

I have a quick question to anyone out there ..

I do not understand the theory behind installing a freeze plug & its effect in reduction of crack initiation.

It would seem to me that the freeze plug provides tensile forces around the circumference of the hole thus initiating rather then reducing crack intiation.
... In the practice of coldworking a hole - I understand how elongating the hole circumference will provide compression stress that have to be overcome .. etc
... could someone explain the principle of freeze plugs in laymans terms?

 

[WKTaylor]

Aconnell.. a bit more complex that it looks...

Fatigue cracks ONLY initiate [from tiny defects] and grow when tension stresses are DYNAMIC [cyclic].

Interference-fit [freeze-fit, press-fit, etc] plugs and stiff bushings generate a static tension field around holes due to interference. As long as dynamic tension stresses around the holes remain AT/BELOW the static tension stress caused by the plug or bushing, then the hole does NOT see any stress changes... therefore NO fatigue cracking growth can occur.

When dynamic stresses fluctuate around the hole ABOVE the static stress due to the plug/bushing, then the effect of the dynamics stresses for fatigue crack growth is reduced by the static tension stress around the hole wall due to the plug: IE: cracks initiate and grow more slowly than a "open or net-fit" hole due to the relatively lower "apparent" tension stress fluctuations.

NOTE: Freeze-fit plugs/bushings are desireable becaues the extreme shrinkage due to cryonic temps allows to the plug/ bushing to be inserted with minimal force and possible hole-wall damage [crack-starters].

CAUTION: stress corrosion cracking prone materials should be carefully evaluated before installling a plug or bushing in significant interference: the possibility for severe SCC failure [static cracking] rises dramaticaly with even low static stresses in many 7XXX-T6 and 2XXX-T3/-T4, very high strength steels, etc. Often the tendency for SCC can be minimized by hole-cold-working... but the static tension field around the hole-wall compression field could STILL induce SCC away from the CX'd hole... if the hole is close to a free-edge [including cutout or non-reinforced hole]... or a severe scratch, dent, nick, corrosion pit, etc... is in the tension field of the CX'd hole.

NOTE: Mandrel/sleeve methods of hole cold working [and in to a lesser degree shot-peening of large holes] adds in the effects of a hole wall [surface] with significant residual compression stresses... and if reinforced by a plug or bushing for "stiffness"... will require significantly higher dynamic tension stresses to see appreciable fatigue grack initiation & growth. Of course, all this comes at a hefty price for cold working tools, expendables, etc... and benefits will probably NOT be allowed [for certification purposes] by FAA-DERS due to possible production "slip-ups on-the-one-critical-hole-out-of-100,000".

CAUTION: on rare occasion cold worked holes or shot-peened surfaces [with surface compression and associated tension fields] can be affected in counter-intuative ways, IE: compression loads close to material yield can in-fact induce compression yield... which can then blow-away all fatigue-cracking initiation/growth predictions.

Regards, WK Taylor

Regards, Wil Taylor

 

[AeroE]

The reason interference fit rapairs or fasteners improve fatigue life is by providing a residual tension field around the hole which causes the hysteresis loop of stress-strain caused by the load to become biased toward tension dominance, reducing the accumulated damage from each load cycle; all that means the stress ratio becomes positive. Review Miner's Rule to understand this effect.